An LCD device includes an LCD panel, polarizers, a backlight device, and an LCD driver circuit. The backlight device includes one or two fluorescent lamps, and a light guide. The light guide receives light from the fluorescent lamp to transmit light of uniform luminance to the LCD panel.
The light guide has a lower surface for performing diffuse reflection and an upper surface for performing diffuse transmission, whereby light of uniform luminance is directed to the LCD panel from the upper surface.
To enhance the uniformity of the light, various technologies have been developed. One of these technologies is shown in Japanese utility model application 61-197685 (Published unexamined utility model application 63-101921), wherein a backlight device includes a diffuse transmission plate facing an LCD panel, a backside reflector and a lamp positioned between the diffuse transmission plate and the backside reflector. A center portion of the diffuse transmission plate has a Fresnel lens arranged parallel to the lamp.
Recently, color LCD devices used in portable personal computers have required a backlight device of high luminance and low power consumption. The high luminance and the low power consumption are inconsistent with each other, because as small a battery as possible is required in a portable personal computer.
Further, recently portable personal computers have been reduced to A4 size. The size of the LCD device, and particularly the size of the backlight device, must also be reduced.
U.S. Pat. No. 4,906,070 discloses a thin flexible film made of a transparent polymeric material including a structured surface and an opposite smooth surface, wherein light striking either surface within certain angular ranges, is totally internally reflected. The structured surface includes a linear array of miniature substantially right angled isosceles prisms arranged side-by-side to form a plurality of peaks of grooves. Viewed in a broad conceptual sense, the structured surface of the film described in this patent may be perceived as resembling the structure of one surface of the optical film of the present invention. However, the optical film of the present invention differs remarkably from the film disclosed in U.S. Pat. No. 4,906,070. Further, the use of the optical film in the particular position in the LCD device according to the present invention is not disclosed or suggested by this patent.